This invention relates to an axial flow steam turbine, and more particularly to such a turbine having improved nozzle blocks for passage of motive steam from a steam inlet to the initial expansion stage of the turbine, and a method for improving the fatigue strength thereof.
Axial flow steam turbines, for the production of electricity generally contain a rotor that is disposed in a casing, and preferably a pair of spaced casings, an outer casing, and an inner casing containing the rotor. This pair of casings enables reduction of thermal gradients and pressure differences across the individual casings so that each casing is free to expand individually. Nozzle chambers are generally disposed within the inner casing which change the direction of inlet steam from a radial to an axial direction and then through nozzle blocks to the blades and vanes of the turbine.
In axial flow steam turbine operations, incoming steam is charged through inlet nozzles to a nozzle ring that contains a plurality of nozzle blocks. These nozzle blocks contain vanes which direct the steam to the control stage or first stage of expansion of the steam. The trailing edges of the nozzles suffer from breaking off or cracking, due to the cycling that is occurring in the structure. Where the steam is exiting at a high velocity, the nozzles tend to crack and fragment, with losses associated therewith. This problem arises because a pressure difference occurs on each side of the nozzle. The problem is exaggerated, however, because hard particles, such as steel flakes, sometimes enter with the steam and increase the cracking problem. The use of highly corrosive-resistent coatings to protect the nozzle vanes is not feasible because of flaking off of such coatings due to different coefficients of expansion. Also, such coatings cannot be used because they reduce the fatigue strength of the component. In order to alleviate the cracking problem, the trailing edges of the nozzle are often made thicker than is necessary or desirable.
One source of the cracking problems relative to the nozzles is the fact that the nozzle is under tension during operation of the turbine. Due to its installation and design, the nozzle carries some of the structural load. If the nozzles were not under tension, the fatigue strength would increase and the trailing edges could be thinned down to a more efficient design.
It is an object of the present invention to provide an axial flow steam turbine wherein the nozzles at the nozzle chamber are under compression rather than tension during operation so as to enhance the fatigue strength of the nozzles.